The biological basis for well-known relationships among conotruncal and neural tube defects is now generally assumed to be found in the common origin of their primordial cells. However, there is no consensus about the mechanism that leads to these defects. A high level of protection of protection is offered for conotruncal, other neural crest, and neural tube defects by folic acid supplementation, implying that an unnamed process of extraordinary importance is sensitively dependent upon an adequate supply of folic acid. For the present proposal, we will test the hypothesis that homocysteine is a teratogen for the conotruncus and other derivatives of the neuroepithelium; and that folic acid supplementation provides protection for embryos by reducing the concentration of homocysteine can induce abnormal development of the conotruncus and other neural crest/neural tube derivatives by acting as an NMDA receptor (NMDA) antagonist. By this mechanism, homocysteine may interact with other NMDA antagonists to exacerbate the teratogenic effect; conversely, it may be predicted that activation of the NMDA would rescue embryos exposed to homocysteine and related compounds. The following specific aims will test this hypothesis. Aim 1, to determine how exogenous NMDA antagonists may interact with homocysteine to exacerbate the disruption of normal development. Aim 2, to measure the degree to which activation of the NMDA may rescue embryos that are treated with homocysteine and related compounds. Aim 3, to analyze changes in gene expression in embryos treated with homocysteine and other NMDA antagonists. SIGNIFICANCE. This proposal offers the first unifying hypothesis regarding a mechanism for a large set of risk factors for abnormal development whose common effect is to inhibit the function of the NMDA. These may include therapeutic drugs, recreation drugs, environmental pollutants, and sequelae of malnutrition. A common mechanism of action would permit these factors to interact in previously unsuspected ways, potentially to exacerbate their respective effects. Effective and comprehensive prevention strategies may be achieved through understanding of such interactive mechanisms.